The use of gas sensors and sensor arrays for odor analysis has attracted a great deal of attention in recent years. The principal goal of such research is to create a technology that can detect a wide range of odors with sufficient reproducibility, selectively, and stability to enable the construction of electronic noses that possess learning, storage, and recognition capabilities. Such systems are expected to be useful in a number of applications including food processing, environmental remediation, agriculture, and medical diagnostics. Organic transistors are being investigated for use in low cost flexible circuitry and in displays. It has been shown that organic transistors also make excellent gas sensors. The sensitivity of some organic transistors and a few gases has been noted in previous work. It is been shown that more information is available from a transistor sensor than an equivalent to chemiresistor sensor. It is an shown that field effect devices with active layers comprised of thin film of a conjugated small molecule, oligomer, or polymer, possess many of the required characteristics of gas sensors. It is been demonstrated that such devices are sensitive to a wide range of vapors at concentrations in the ppm range. The large variety of semiconductor materials available and the degrees of freedom available in modifying their molecular and morphological structures enable the construction of sensor arrays that could detect odors through pattern recognition.
The basic structure of the field effect sensor shown in FIG. 1(a) of “Electronic Sensing of Vapors with Organic Sensors” Applied Physics Letters, Volume 78, Number 15, Apr. 9, 2001. The field effect sensor consists of a thin film (of the order 10-100 nm) of an active semiconductor deposited by either a vacuum sublimation or solution-based techniques on dielectric-coated conductor. Gold electrodes are evaporated over the semiconductor with spacing of 200 um. The devices is biased so that the channel has a field-induced charge with densities in the range of 1012-1013 cm−2. The functioning of such field effect devices is described in the literature. Measurements were also made with zero gate bias. The morphology of semiconductor materials used in the study is polycrystalline with grain size in the 10-100 nm range.
While the organic transistors demonstrate a change in drain to source current when its semiconductor channel is exposed to selected gases, their sensitivity is low and it is difficult to “reset” the device by clearing out the trapped charges after an exposure to an analyte gas. Additionally, in previous work on organic FET sensors, the active organic transistor channel has a dual role: sensing as well as transduction. This dual role may cause reliability problems. Therefore there is a need for a new gas sensor device that uses a organic field effect transistor (FET) device combined with a silicon semiconductor field effect device that is more sensitive and has the ability to be electronically reset. There is also a need for a gas sensor device where the sensing takes place in the organic FET and transduction takes place in a parallel integrated silicon FET device.